1. Field of the Invention
The invention relates to a differential amplifier comprising a first and a second transistor of the same conductivity type, whose emitters are each connected to a common point via a first resistor, which first resistors are substantially equal to each other, a third transistor which is of the same conductivity type as the first and the second transistor and whose emitter is coupled to the common point, and a current source for supplying the quiescent current of the amplifier, which source is connected to the common point.
2. Description of the Related Art
An important characteristic of differential amplifiers is the so-called "slew rate". This is to be understood to mean the maximum possible rate of change of the output signal of the amplifier in the case of a capacitive load of the differential amplifier. This slew rate is proportional to the maximum current that can be supplied by the collectors of the first and the second transistor, which current is substantially equal to the quiescent current supplied by the current source in the common emitter circuit. If negative feedback is applied by arranging resistors in the emitter lines the transconductance becomes substantially inversely proportional to the resistance value of these resistors, so that the gain becomes substantially independent of the quiescent current. The slew rate may then be increased by increasing the quiescent current. However, the emitter resistors can be made identical to each other with a limited accuracy only, so that an increase of the quiescent current loads to a higher offset voltage and also to a larger offset-voltage drift. Especially for low input voltages this is a disadvantage. A known solution to this problem is to cause the quiescent current of the differential amplifier to increase when the input voltage increases. In a differential amplifier disclosed in U.S. Pat. No. 4,002,993 this is achieved by means of a third transistor whose emitter is connected to the junction point of the emitter resistors. This third transistor is driven by two transistors of a conductivity type opposite to that of the first, the second and the third transistor, the bases of the two transistors being connected to the emitters of the transistors of the differential amplifier and the emitters of the two transistors being connected to the base of the third transistor and to a current source. As a result of this configuration the third transistor drains a substantial portion of the quiescent current of the differential amplifier when the input signals of the differential amplifier are small, the portion of this quiescient current drained by this transistor decreasing as the input signals become larger.